Micro-electro-mechanical system (MEMS) transducers such as MEMS microphones are increasingly finding application in portable electronic devices such as mobile telephones, laptop and tablet computers, audio and video players, personal digital assistants (PDAs) and wearable devices such as smart watches, at least in part due to the small size of such transducers.
Transducers such as capacitive microphones or pressure sensor devices formed using MEMS fabrication processes typically comprise an electrode that is moveable with respect to a fixed electrode in response to incident acoustic or pressure waves, such that the fixed electrode and the moveable electrode together form a variable capacitance. The moveable electrode may, for example, be supported by a flexible membrane. In use a first one of the electrodes may be biased by a relatively high, stable bias voltage VBIAS, which may be of the order of 12V, whilst the other electrode is biased to another fixed voltage VREF, typically ground, via a very high impedance, for example, of the order of 10 GΩ. Acoustic or pressure waves incident on the transducer will cause displacement of the moveable electrode with respect to the fixed electrode, thus changing the spacing between these electrodes and hence the inter-electrode capacitance. As the second electrode of the transducer is biased via a very high impedance, these changes in capacitance cause an output signal voltage to appear at an output terminal of the transducer. The output signal voltage for normal sound pressure levels is of the order of millivolts and the capacitance of the MEMS transducer is of the order of a picofarad, so the transducer is typically connected to a low noise amplifier arrangement to buffer and amplify the signal.
To provide protection the MEMS transducer is typically housed within a package. The package effectively encloses the MEMS transducer and can provide environmental protection and may also provide shielding from electromagnetic interference (EMI) or the like. The package also provides at least one external connection for outputting an output signal to downstream circuitry. For microphones, pressure sensors and the like the package typically has a sound port to allow transmission of sound waves to or from the transducer within the package, and the transducer may be configured so that the flexible electrode is located between first and second volumes, i.e. spaces or cavities that may be filled with air, and which are sized sufficiently that the transducer provides the desired acoustic response. The first volume, sometimes referred to as a back volume, on a first side of the flexible electrode, allows the flexible electrode to move freely in response to incident sound or pressure waves, and this back volume may be substantially sealed. The sound port acoustically couples to a second volume on a second side of the flexible electrode, which may sometimes be referred to as a front volume. It will be appreciated by one skilled in the art that for MEMS microphones and the like the first and second volumes may be connected by one or more flow paths, such as small holes in the flexible electrode that are configured so as to present a relatively high acoustic impedance at the desired acoustic frequencies but which allow for low-frequency pressure equalisation between the two volumes to account for pressure differentials due to temperature changes or the like.
The package may also contain support circuitry on the same or a separate semiconductor die as the moveable electrode. A function of the support circuitry is to measure the output signal voltage of the transducer. The support circuitry may also provide one or more audio processing functions such as filtering, equalisation and the like. The support circuitry may also provide bias to the electrodes, analogue to digital conversion, analogue or digital signal conditioning, an analogue or digital output interface, and/or other functions.
It has been found that the sensitivity of MEMS transducers such as capacitive microphones (which may be defined as the ratio of the output signal voltage of the MEMS transducer to the input acoustic pressure) can change over time. This is known as sensitivity drift. In some cases the sensitivity of the transducer increases over time, whilst in other cases the sensitivity of the transducer may decrease over time. As will be appreciated, sensitivity drift in a MEMS transducer is undesirable, as it can cause inconsistent performance of a device in which the MEMS transducer is incorporated. Thus, a need exists for a mechanism for detecting sensitivity drift in a MEMS transducer such as a MEMS capacitive microphone and compensating for any detected drift.